A wavelength-division multiplexing (WDM) transmission system is capable of transmitting a large amount of information by multiplexing signal lights modulated by various modulation methods. Methods for modulating signal lights for the WDM system include an on-off keying (OOK) modulation method and an optical phase modulation method. The OOK modulation method is a method in which information is transmitted and received by switching light between on and off, and is used for a transmitter-receiver having a bit rate of about 10 Giga-bits per second (Gbps). In contrast, the optical phase modulation method is a method in which information is transmitted and received by changing the phase of light, and is used for a transmitter-receiver having a bit rate of about 40 Gbps. The optical phase modulation method has a noise tolerance and a variation tolerance greater than the OOK modulation method.
The optical phase modulation method includes a differential phase-shift keying (DPSK) modulation method and a differential quadrature phase-shift keying (DQPSK) modulation method. The DPSK modulation method includes various types of modulation methods such as, for example, non-return-to-zero DPSK (NRZ-DPSK). Furthermore, a modulation method called a “dual-polarization quadrature phase-shift keying (DP-QPSK)” modulation method has been developed in recent years. In the DP-QPSK modulation method, polarized waves are multiplexed in order to transmit and receive information at about 100 Gbps. A dual-polarization-multiplexed signal includes a DP-QPSK modulation signal, for example. A non polarization-multiplexed signal includes a DPSK modulation signal and an intensity modulation signal, for example.
In the WDM transmission system, a signal light is relayed by using an amplifier called an “erbium-doped fiber amplifier (EDFA)”. The EDFA is an amplifier in which a fiber doped with a rare earth ion is used as an optical amplification medium. In addition, the EDFA collectively amplifies signal lights having wavelengths in a band extending from 1530 nm to 1625 nm. In particular, in the band of wavelengths amplified by the EDFA, a band extending from 1530 nm to 1565 nm is called a “C band”, and a band extending from 1565 nm to 1625 nm is called an “L band”. Japanese Laid-open Patent Publication No. 7-107069 is an example of the related art.
In the case of the above-described EDFA, however, a phenomenon called “polarization hole burning (PHB)” occurs, thereby deteriorating the transmission quality. PHB is a phenomenon in which the EDFA obtains polarization-dependent gain (PDG) in accordance with the polarization status of a signal light input to the EDFA. Due to the effect of PHB, gain for amplified spontaneous emission (ASE), which is a noise component generated in the EDFA, may be large depending on the status of polarized waves of the signal light.